The present invention relates generally to surge arresters. More particularly, the invention relates to a new design for the internal components of surge arresters. Still more particularly, the invention relates to a new combination of metal oxide varistors (MOV's) and spark gap assemblies in which the MOV elements conduct low magnitude, steady-state current through the MOV along a first current path, and conduct the higher magnitude impulse or surge currents through the MOV along a separate and distinct path.
Under normal operating conditions, electrical transmission and distribution equipment is subject to voltages within a fairly narrow range. Due to lightning strikes, switching surges or other system disturbances, portions of the electric system may experience momentary or transient voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. Left unprotected, critical and costly equipment such as transformers, switching apparatus, and electrical machinery may be damaged or destroyed by such overvoltages and the resultant current surges. Accordingly, it is routine practice within the electrical industry to protect such apparatus from dangerous overvoltages through the use of surge arresters.
A surge arrester is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert the overvoltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. When caused to operate, a surge arrester forms a current path to ground having a very low impedance relative to the impedance of the equipment that it is protecting. In this way, current surges which would otherwise be conducted through the equipment are instead diverted through the arrester to ground. Once the transient condition has passed, the arrester must operate to open the recently-formed current path to ground and again isolate or "reseal" the distribution or transmission circuit in order to prevent the nontransient current of the system frequency from "following" the surge current to ground, such system frequency current being known as "power follow current." If the arrester did not have this ability to interrupt the flow of power follow current, the arrester would operate as a short circuit to ground, forcing protective relays and circuit breaker devices to open or isolate the now-shorted circuit from the electrical distribution system, thus causing inconvenient and costly outages.
Conventional surge arresters typically include an elongated enclosure or housing made of an electrically insulating material, a stack of voltage-dependent, nonlinear resistive elements retained within the enclosure, and a pair of electrical terminals at opposite ends of the enclosure for connecting the arrester between a line-potential conductor and ground. The nonlinear resistive elements are chosen to have a higher resistance at the normal steady-state voltage and a much lower resistance when the arrester is subjected to high magnitude transient overvoltages. Depending on the type of arrester, it may also include one or more spark gap assemblies housed within the insulative enclosure and electrically connected in series with the nonlinear resistive elements.
Present-day surge arresters are typically one of two basic types and are generally classified according to the type of nonlinear resistive elements they contain. The first type of conventional arrester is commonly referred to as the series gapped silicon carbide (SiC) arrester. The nonlinear resistive elements in this arrester are relatively short cylindrical blocks of silicon carbide which are stacked one atop the other within the arrester housing in series with spark gap assemblies which are generally resistance graded gap assemblies. A resistance graded gap assembly comprises a resistor electrically in parallel with the spark gap and usually includes one or more resistors in series with the gap. This network of resistors is employed to control the voltage level at which the spark gap will begin to conduct. The second type of arrester commonly used today is know as the gapless metal-oxide varistor (MOV) arrester. In this type of arrester, the nonlinear resistive elements comprise disks formed of a metal oxide compound which are again stacked within the arrester housing in series.
In both types of prior art arresters, the voltage-current relationship for the nonlinear elements is expressed as I=kE.sup.n, where I is arrester current, k is a constant, E is the arrester voltage, and n is the nonlinear exponent or coefficient. The older series gapped SiC arrester uses low exponent silicon carbide blocks in series with low exponent nonlinear graded gaps, the exponent n of both elements being less than 10 and typically being within the range of 4 to 5 at the operating or steady state voltage. The more modern MOV arrester typically uses only high exponent nonlinear elements of the metal-oxide variety and, as described below, does not require series gap assemblies to operate properly as is the case of SiC arresters. In the case of MOV arresters, the exponent n is usually greater than 10 and typically about 20 or greater at the steady-state system voltage.
Because of the different degrees of nonlinearity of the resistive elements employed in silicon carbide and MOV arresters, these arresters differ in structure and operation. The silicon carbide blocks are designed to provide a very low resistance to surge currents, but a higher resistance to the 60 hertz power-follow current which continues to flow through the arrester after the transient condition has passed. Despite the higher resistance, the silicon carbide blocks will still conduct large currents at the normal, steady-state line-to-ground voltage. Accordingly, gap assemblies are employed in series with the silicon carbide blocks. As a transient overvoltage condition ceases, the resistance of the silicon carbide blocks increases so as to limit the magnitude of the power follow current. The reduced current flow and the corresponding decrease in the voltage across the spark gaps provide the gap assemblies the opportunities to open the current path to ground and thus "reseal" the power circuit after the surge has passed. This type arrester has been in use for many years and is described in many earlier patents, such as U.S. Pat. Nos. 4,161,763 and 4,174,530.
With an MOV arrester, the MOV elements provide either a high or a low impedance current path between the arrester terminals depending on the voltage appearing across the varistor elements themselves. More specifically, at the power system's steady-state or normal operating voltage, the varistors have a relatively high impedance. As the applied voltage is increased, gradually or abruptly, the varistors' impedance progressively decreases. When the voltage appearing across each varistor reaches the elements' breakdown voltage, the varistor impedance dramatically decreases, and the varistors become highly conductive. Accordingly, if the arrester is subjected to an abnormally high transient overvoltage, such as may result from a lightning strike, for example, the varistor elements become highly conductive and serve to conduct the resulting transient current to ground. As the transient overvoltage and resultant current dissipate, the varistor elements' impedance once again increases to a very high value, thereby reducing the current through the MOV arrester to a negligible flow and restoring the arrester and electrical system to their normal, steady-state condition. A variety of MOV arresters have been described in many earlier patents, such as U.S. Pat. Nos. 4,930,039 and 4,240,124.
The series gapped SiC arresters suffer from a variety of undesirable traits. First, because the SiC elements are highly conductive at normal operating voltages, the gap assemblies are required to support the full system line-to-ground voltage over the life of the arrester, the SiC elements being used only to limit current which, in turn, assists the gaps in returning to their non-conductive mode during a discharge operation as described above. Because the gap assemblies must support the full line-to-ground voltage, SiC arresters are typically comprised of many such assemblies, each of which must withstand its proportionate share of the voltage. This type of construction results in more consistent impulse or spark-over characteristics than can be achieved through the use of MOV arresters; however, the undesirable result is that the design yields higher than desired impulse protective characteristics.
Another deficiency characteristic of the series gapped SiC arrester is that its high current discharge characteristic and the power follow current levels are both controlled by the same nonlinear elements, i.e., the SiC elements. To achieve lower high current discharge voltages, it is desirable to have silicon carbide elements with a low resistance. Yet, to provide lower levels of power-follow current, it is desirable to have silicon carbide elements with a high resistance. Due to these diametrically opposed requirements of the same components, design compromises have resulted in less than desirable protective characteristics. Still another inherent problem with the series gapped SiC arrester is its comparatively large size and weight.
The MOV arrester was developed to eliminate the undesirable impulse characteristic of the series gapped silicon carbide arrester. In the MOV arrester, the nonlinear MOV elements eliminate the need for a series gap by remaining highly non-conductive at normal, steady state system voltages. As the voltage applied to the arrester is increased, the MOV element, which is a semiconductor, gradually begins to conduct, without a disruptive discharge as is characteristic with the series gapped SiC arrester. This switch-like characteristic enables the MOV arrester to shunt all fundamental transient overvoltage energies to ground. The inherent problem with this type of arrester, however, is that both the turn-on or breakdown voltage and the high current discharge voltage are controlled or dictated by the same nonlinear elements. It is desirable for the arrester to have higher breakdown voltages so that transient overvoltages having a lower, nondestructive magnitude do not result in conduction through the MOV elements. At the same time, it is also desirable for the arrester to have lower high current discharge voltages to provide better equipment protection. Again, as with the series gapped SiC arrester, two diametrically opposed requirements of the same component result in a compromise of characteristics. In some cases the discharge voltage capability of the MOV arrester is compromised. In other instances, the arrester's ability to withstand a temporary, relatively low overvoltage condition, defined as the arrester's temporary overvoltage capability, is reduced.
More recently, a hybrid arrester has been developed which combines the gap assemblies previously used in the silicon carbide gapped arresters with the MOV elements of the metal oxide varistor arrester. Such hybrid arrester is described in co-pending U.S. patent application, Ser. No. 07/420,069, and in the publication entitled New Surge Arrester Technology Offers Substantial Improvement in Protection and Reliability as presented to the SEE Overhead Committee, Annapolis, Md., May 10, 1990, such written disclosures being incorporated herein by reference. The hybrid arrester has been shown to have superior performance characteristics as compared to both the SiC gapped arresters and the MOV arresters.
Despite the advances made by the hybrid arrester, further technological advances would be welcomed by the industry. Specifically, the resistance graded gap structures used in the silicon carbide gapped arrester and in the hybrid arrester must be precisely matched. Further, assembly of the complicated gap structures in the arrester is tedious and thus costly. An arrester having similar or improved characteristics as compared to the hybrid arrester, but without the disadvantages associated with the resistance graded gap structures would be a welcomed addition to the art. It would further be desirable to decrease the volume of expensive MOV material currently required in the MOV and hybrid arresters.